1. Field of the Invention
The present invention relates to a method for purifying tungsten hexafluoride of high purity, and more specifically relates to a method for purifying tungsten hexafluoride to high purity and removing molybdenum hexafluoride, which contaminates tungsten hexafluoride, both cheaply and efficiently.
2. Description of the Related Art
Known methods used for purifying tungsten hexafluoride are reactions of chlorides of tungsten with fluorides such as HF and AsF3 and reactions of metal tungsten with fluorine.
If it is assumed that impurities, other than molybdenum, which have contaminated tungsten hexafluoride produced by these methods are those of chlorides and fluorides, these impurities can be readily separated and removed from tungsten hexafluoride by using a difference in boiling point and vapor pressure of tungsten hexafluoride from those impurities. However, molybdenum has similar properties to those of tungsten because molybdenum is a congener of tungsten and is therefore difficult to separate and remove by distillation.
As the method for removing molybdenum from tungsten, a method is described where a metal piece makes contact with tungsten hexafluoride at a temperature between 100° C. and 500° C. (Japanese Patent No. 2848717). Using this method, it is necessary for all of the tungsten hexafluoride to be converted to tungsten trifluoride and a distillation tower for purification therefore has to keep the temperature in a range from 100° C. to 500° C. This method is therefore neither suitable nor effective when operating costs are taken into consideration.
Moreover, the above described technology carries out heating to a high temperature ranging from 100° C. to 500° C. to cause decomposition of WF6 by installation of a packed tower made of SUS (stainless steel.)
The present invention provides a method for purifying tungsten hexafluoride of high purity, wherein production of tungsten hexafluoride of higher purity than in the related art is possible in a straightforward manner and at a low cost.